How are these two higher-order functions different?

Question

The following two higher-order functions yield the same result, but I'm having trouble understanding the difference between the two. In fact, I don't understand 1). How can a function have two sets of ()?

1)

def sum(f: Int => Int) (a: Int, b: Int) = {
    def loop(a: Int, acc: Int) : Int =
        if (a > b) acc
        else loop (a + 1, f(a) + acc)
    loop (a, 0)
}

2)

def sum(f: Int => Int,  a: Int, b: Int) = {
      def loop(a: Int, acc: Int) : Int =
        if (a > b) acc
        else loop (a + 1, f(a) + acc)
      loop (a, 0)
}

Scala functions can have multiple parameter lists (as in your first defintion of `sum`). See http://docs.scala-lang.org/tutorials/tour/currying.html — jub0bs, Mar 19 '16 at 13:59
See http://stackoverflow.com/questions/36314/what-is-currying — Kevin Meredith, Mar 19 '16 at 14:00

Luka Jacobowitz · Accepted Answer · 2016-03-19T14:20:28.253

3

The first function is curried. Meaning you can partially apply it more easily. If you only use the first parameter list the function returns another function with the signature (Int, Int) => Int.

This is really useful if you need to pass a function with a specific function signature to higher order functions like map or reduce.

In other functional programming languages like Haskell, all functions are curried by default. I wrote a whole blog post on this topic in case you're interested. You can read it right here.

edited Mar 19 '16 at 14:20

answered Mar 19 '16 at 14:03

Luka Jacobowitz

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Can you post your blog link? Thanks – johnsam Mar 19 '16 at 14:05
So only 2) is high order function. And 1) is not? 1) is currying instead? – johnsam Mar 19 '16 at 14:23
No, a higher order function is a function that takes a function as a parameter, like `map`, `filter` or `reduce`. So 2) is a normal function and 1) is the same function, but curried. :) – Luka Jacobowitz Mar 19 '16 at 14:27
Correction, both 1) and 2) are higher order functions, but only 1) is curried :) – Luka Jacobowitz Mar 19 '16 at 14:46

How are these two higher-order functions different?

1 Answers1